1. Field of the Invention
This invention generally relates to a process for the pyrolysis of 1,2-dichloroethane to form vinyl chloride monomer in a pyrolysis furnace. More particularly, it relates to a method and means for recovering heat from such a pyrolytic process by adding a heat exchanger to the furnace.
2. Prior Art
As is well known in the art, vinyl chloride monomer (to be abbreviated as VCM, hereinafter) is generally produced by pyrolyzing 1,2-dichoroethane (to be abbreviated as EDC, hereinafter) in a pyrolysis furnace. The pyrolysis furnace includes a convection heat transfer conduit extending in an upper convection heat transfer region, a radiation heat transfer conduit extending in a lower radiation heat transfer region, and means for heating the radiation heat transfer conduit. Liquid EDC is fed into the convection heat transfer conduit where EDC is preheated for vaporization, and then channeled into the radiation heat transfer conduit where the gaseous EDC is pyrolyzed to generate VCM and hydrogen chloride.
In this process, for the purpose of suppressing formation of by-product impurities and formation of carbon due to excessive decomposition, pyrolysis reaction is controlled to such an extent that about 50 to 60% of the EDC feed is decomposed. Then the decomposition gas exiting from the pyrolysis furnace mainly contains VCM, hydrogen chloride, and EDC. The gas available at the pyrolysis furnace outlet (to be referred to as decomposition gas, hereinafter) is generally at a temperature of about 450 to 550.degree. C.
Various attempts were made in the prior art to recover the potential heat that the hot decomposition gas possesses for effective utilization of heat. For example, Japanese Patent Application Kokai (JP-A) No. 129233/1980 discloses a method for recovering potential heat from decomposition gas in an industrially acceptable manner. This method uses a heat exchanger for recovering heat from the decomposition gas and utilizes the recovered heat for heating EDC feed. When the high temperature the decomposition gas possesses is utilized for preheating and evaporating (or vaporizing) EDC, the fuel consumed in the pyrolysis furnace can be saved about 20 to 25%. This heat recovery method is quite advantageous from economic and environmental aspects.
The method of JP-A 129233/1980 for recovering potential heat in decomposition gas by means of a heat exchanger and utilizing the recovered heat for heating EDC feed is described in detail. After EDC is fed into a convection heat transfer region of the pyrolysis furnace for preheating EDC, the EDC is taken out of the furnace and fed to a heat exchanger which also receives the decomposition gas that has been heated hot, whereby heat is recovered from the decomposition gas. EDC is heated with the recovered heat for partial evaporation or vaporization. Thereafter, gaseous EDC is separated from the partially evaporated or vaporized EDC. The gaseous EDC is fed back to a radiation heat transfer region of the pyrolysis furnace. The heat exchanger used in this method is a single tube type heat exchanger having an inner tube extended in a jacket in a serpentine manner. Decomposition gas flows through the inner tube while liquid EDC is contained in the jacket. Since liquid EDC remains in the jacket of the single tube type heat exchanger, the amount of gaseous EDC taken out of the jacket is not always coincident with the amount of liquid EDC fed thereto. When the gaseous EDC is fed to the pyrolysis furnace, its flow rate must be controlled. Also the surface of liquid EDC in the jacket must be controlled. A new control loop is thus required. Undesirably, the control system becomes complex and the instrumentation cost is increased.
The method of JP-A 129233/1980 has another problem that since liquid EDC is evaporated or vaporized in the jacket of the heat exchanger, carbonaceous scale or carbon scale is likely to deposit on the inner wall of the jacket in contact with EDC. Undesirably the carbonaceous scale deposited on the inner wall reduces heat transfer. It is thus necessary to remove the carbonaceous deposit from the inner wall.